Closed loop catheter coolant system

ABSTRACT

A coolant system for a cryoablation or treatment probe such as a mapping or ablation catheter, or a treatment wand, includes a compressor and condenser having a low pressure inlet side and a high pressure outlet side, wherein the outlet side passes through a heat exchanger and is cooled by the inlet side and conditioned for injection to a catheter inlet. A vacuum return system connectable to the catheter outlet draws thermally expended coolant from the catheter and returns it to the low pressure inlet side. A motorized pressure regulator between the heat exchanger and the catheter inlet determines the pressure of coolant passing into the catheter and thus regulates the cooling rate for a selected mapping or ablation regimen. The low pressure compressor inlet supply preferentially conditions the pressurized coolant to ambient temperature or lower before injection into the catheter, allowing the coolant to travel through the body at ambient before expansion in the tip.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/115,213, filed Apr. 01, 2002, by Jean-Pierre Lalonde, et al, entitledCLOSED LOOP CATHETER COOLANT SYSTEM, now U.S. Pat. No. 6,682,525, whichapplication is a continuation of U.S. patent application Ser. No.09/489,646, filed Jan. 24, 2000, by Jean-Pierre Lalonde, et al.,entitled CLOSED LOOP CATHETER COOLANT SYSTEM, now issued U.S. Pat. No.6,383,180 B1, which application is related to and claims priority fromU.S. Provisional Application Ser. No. 60/117,175, filed Jan. 25, 1999,the entirety of all of which are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

BACKGROUND OF THE INVENTION

The present invention relates to a coolant system for a catheter ortreatment wand used for cryotreatment of tissue. In particular, thecoolant system is of the type which connects to a catheter and pumpscoolant through the catheter to chill the tip of the catheter forstunning or ablating tissue, such as cardiac wall tissue, for mapping ortreatment purposes. The invention also contemplates ablation systems fornon-cardiac tissue, employing, for example, a hand-held treatment wandrather than a catheter delivery system. For purposes of illustrationherein, the discussion will be directed primarily to endovascularablation catheters of the type available for treating cardiacarrhythmias.

A number of cooled catheter systems have been developed for treatingtissue in a cardiac setting, either to cool the tissue sufficiently tostun it and allow cold mapping of the heart and/or confirmation ofcatheter position with respect to localized tissue lesions, or to applya more severe level of cold to ablate tissue at the site of the catheterending. In general, the range of treatments which may be effected by acryocatheter is comparable to the range of applications for RF orthermal ablation catheters, and in particular, these instruments may beconfigured to achieve either small localized ball shape lesions at thetip of the catheter, or one or more elongated linear lesions extending alength of several centimeters or more along the tip. The latter form oflesion is commonly used to achieve conduction block across a region ofthe cardiac wall so as to sever a re-entrant pathway, preventingconduction across the region, in order change the cardiac signal pathtopology, for example, to eliminate a re-entrant pathway responsible foratrial fibrillation or a tachycardia.

In general, when used for endovascular access to treat the cardiac wall,catheters of this type, in common with the correspondingearlier-developed radio frequency or electrothermal ablation catheter,must meet fairly demanding limitations regarding their size,flexibility, and the factors of strength, electrical conductivity andthe like which affect their safety and may give rise to failure modes inuse. These constraints generally require that the catheter be no largerthan several millimeters in diameter so as to pass through the vascularsystem of the patient to the heart. Thus, any electrodes (in the case ofmapping or RF/electrothermal ablation catheters), and any coolantpassages (in the case of cryocatheters) must fit within a catheter bodyof small size.

A number of different fluids have been used for the coolant component ofprior art cryotreatment catheters. Among these may be mentioned a coolliquid such as a concentrated saline solution or other liquid ofsuitably low freezing point and viscosity, and of suitably high thermalconductivity and heat capacity, or a liquified gas such as liquidnitrogen. In all such constructions, the coolant must circulate throughthe catheter, thus necessitating multiple passages leading to thecooling area of the tip from the catheter handle.

Furthermore, conditions of patient safety must be considered, raisingnumerous problems or design constraints for each particular system. Thusfor example, a high pressure may be required to circulate sufficientcoolant through the catheter body to its tip and back, and the overalldesign of a catheter must be such that fracture of the catheter wall orleakage of the coolant either does not occur, or if it occurs, isharmless. Further, for an endovascular catheter construction, thepresence of the coolant and circulation system should not substantiallyimpair the flexibility or maneuverability of the catheter tip and body.

To some extent these considerations have been addressed by using a phasechange material as the cryogenic fluid, and arranging the catheter suchthat the phase change, e.g., from a liquid to a gas, occurs in thetreatment portion of the catheter tip. Another possible approach is toemploy a pressurized gas, and configure the catheter for cooling byexpansion of the gas in the tip structure. However, owing to the smallsize that such a catheter is required to assume for vascular insertion,or the awkwardness of handling a cryogenic treatment probe generally,the design of a safe and effective coolant circulation system whichnonetheless dependably provides sufficient cooling capacity at a remotetip remains a difficult goal.

Among other common problems to be addressed while providing adequatethermal capacity, may be noted the leakage problem mentioned above, theproblem of effectively preventing the catheter as a whole from beingexcessively cold or damaging tissue away from the intended site, and theproblem of conduit or valve blockage owing for example to ice particlesand the like.

Accordingly, it would be desirable to provide a coolant system whichconveniently attaches to a cryocatheter.

It would also be desirable to provide a closed loop coolant system whichinjects and retrieves the coolant from the catheter to allow continuousoperation without leakage into the environment or other loss of coolant.

It would further be desirable to provide a closed loop treatment systemwhich precisely controls ablation and treatment regimens by conditioningthe coolant supply side of a closed loop.

SUMMARY OF THE INVENTION

These and other desirable features are obtained in a coolant system fora cryoablation or treatment probe such as a mapping or ablationcatheter, or a treatment wand, which includes a compressor and condenserhaving a low pressure inlet side and a high pressure outlet side,wherein the outlet side passes through a heat exchanger to be cooled bythe inlet side and conditioned for injection to a catheter inlet, andfurther comprising a vacuum return system connectable to the catheteroutlet to cause thermally expended coolant from the catheter to flowthrough the vacuum system and be returned to the low pressure inletside. A motorized pressure regulator between the heat exchanger and thecatheter inlet determines the flow rate of coolant passing into thecatheter and thus regulates the cooling power for a selected mapping orablation regimen.

Preferably, the low pressure inlet operates at relatively lowtemperatures so that heat exchange conditions the relatively warmpressurized coolant to ambient temperature or colder before injectioninto the catheter, allowing the coolant to travel at near ambienttemperature to the tip before expansion and cooling to perform mappingor ablation as appropriate. In a preferred embodiment, a coolantreservoir feeds into the low pressure inlet side and receives a returnflow of excess fluid from a branch off the outlet side of thecompressor. The vacuum return assures that coolant does not leak intothe blood stream, and preferably various check valves and bypass valvesoperate in the event of pressure buildup to return fluid to the inlet orsupply loops. The coolant mixture preferably has a boiling point ofapproximately −60° Fahrenheit at about one atmosphere, and may becompressed to several hundred psi. The entire system is amenable tomicroprocessor control for providing ablation cooling cycles to operatethe catheter tip in accordance with a selected protocol, and foreffecting system functions such as recharging and venting of the coolantsupply, and shutting down during nonuse or upon occurrence of a faultcondition.

BRIEF DESCRIPTION OF DRAWINGS

These and other features of the invention will be understood byreference to the description below, read in light of the prior arttogether with illustrative figures, wherein:

FIGS. 1 and 1A illustrate a cryocatheter treatment system andcryocatheter;

FIG. 2 is a schematic representation of a coolant system in accordancewith one embodiment of the present invention for use with the catheterof FIG. 1; and

FIG. 3 is a detailed schematic of another implementation of the coolantsystem of the present invention.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows a cryogenic treatment system 100 illustrating the generalelements thereof. System 100 includes a treatment catheter 110 having ahandle 110 a, a treatment console 120 and number of connecting lines 115which include signal lines for any cardiac monitoring or mappingfunctions as well as a coolant injection line 115 a and a coolant returnline 115 b. As illustrated, the console includes a display screen 120 awhich may, for example, show both cardiac electrical signals and variousstatus and control screens related to setting or reporting the coolingfunctions of the catheter or the ablation regimens being administeredtherewith.

FIG. 1A shows in slightly greater detail a catheter 110 used in a systemin accordance with the present invention. As shown, the handle 110 a isequipped with input ports for an electrical connector 111, a coolantinjection tube connector 112, and a return tube connector 113. Theseconnect via various internal junctions or tubes passing through thehandle to provide these three functions to the distal tip of thecatheter. The handle may also include various control assemblies, e.g.,switches or valves, as well as safety detection or shut down elements(not illustrated).

Leading from the handle 110 a is an elongated catheter body 110 b whichextends to the catheter tip 110 c, illustrated in enlarged detail toshow a representative structure thereof. As shown, in catheter tip 110 cthe coolant enters through a central tube 1 and exits via a nozzle 2 atthe end of the tube to expand in a small contained region forming achamber 3 at the tip of the catheter. In the illustrated construction,the tube 1 runs concentrically within an outer tube (not numbered)thereby forming an annular return space 4 surrounding the supply tube 1and extending back to the fluid return connector 113 of the handle. Asdiscussed further below, the return passage for expended coolant is avacuum passage, thus assuring that leakage into the blood stream cannotoccur.

The location of chamber 3 defines the cooling region of the cathetertip. In the illustrated embodiment this is a short chamber less than acentimeter long located at the very tip of the catheter. Also shown area thermocouple 5 positioned within the tip to sense tip temperature, anda plurality of electrodes including a tip electrode 7 a and one or morering electrodes 8 a, 8 b . . . which are positioned near the tip for usein mapping and/or detecting cardiac signals. In other embodiments, thechamber 3 defined at the tip of the catheter may be an elongated chamberseveral centimeters in length for defining a coolant chamber effectiveto form linear lesions when placed in contact with tissue such as thecardiac wall. For the linear embodiment, multiple expansion nozzles, aperforated inlet tube end segment, or other variation in theconstruction of the coolant supply line may be used to assure a highrate of cooling along the full length of the expansion chamber.Furthermore, the chamber wall may be very thin, or formed with a metalsleeve or cap to achieve high heat transfer rates. Other structureswithin the catheter may include torque or steering wires, or otherelements conventional in the art for navigation of the catheter pastbranch points in vessels, and for urging the catheter tip into contactwith a wall once its position is confirmed.

As will be understood from the above, the task of the console is toprovide coolant at the tip region in sufficient quantity and for timeseffective to create the desired lesions. The nature and depth of thelesions created will depend on a number of factors, including thetemperature attained in the adjacent tissue, as well as the nature ofthe cooling cycle by which that temperature is attained. In general whenthe tissue attains an extremely low temperature, or a temperatureeffective to create ice crystals within tissue cells, the tissue damagewill be irreversible, resulting in effective ablation at the contactedsite. The actual cooling rates achieved at the tip will depend to alarge extent on the area of contact with the tissue as well as theconductive properties of the adjacent tissue and the structure andgeometry of the catheter in addition to the nature of coolant flowpassing through the catheter tip. In the present system applicantcontrols the latter quantity, as discussed more fully below, byproviding a controller in which the flow of a phase change coolantsupplied to the tip is varied to directly control the amount of coolingpower available during an ablation cycle. In addition, the primarycooling effect is achieved by expansion of coolant at the inlet nozzle 2as it enters chamber 3.

While not illustrated, one or more electrical sensing elements inaddition to the thermocouple may be provided at various places withinthe catheter to provide useful feedback or emergency control functions.For purposes of the present patent application, such functions will notbe further discussed. However, if provided they may be positioned in adiscrete cooling system, which for purposes of illustration may beconsidered to lie entirely within the console 120, or be externalthereto, but in any case to function in relation to the coolant supplyelements which will now be described below.

FIG. 2 illustrates one embodiment of a cooling system in accordance withthe present invention configured to connect to the inlet and returnports 112, 113 of the catheter 110 (FIG. 1A). As shown, the coolantsystem 120 includes a coolant supply 30, a coolant conditioner 40, acoolant control 50 and a coolant return section 60. The control section50 connects to the inlet 112 of the injection catheter, for example by asupply tube, while the return system 60 connects to coolant return port113. These are illustrated as separate connections, but as discussedmore fully below, they may be implemented with a single vacuum-jacketedline with a quick connect coupler, or other specialized connection whichallows a single coupling to the catheter handle for all coolantfunctions. Similarly, electrical connections may be incorporated in sucha single conduit, or may be provided as separate signal cabling.Operation of the coolant system 120 will be most fully understood from adetailed discussion of each of the subassemblies 30, 40, 50, 60.

In general terms, the coolant system has a coolant conditioning section40 with a compressor that provides a conditioned phase change coolant atelevated pressure to the control section 50, which, in turn, regulatesthe supply of coolant provided to the inlet of the catheter. The returnsection 60 includes a vacuum pump which continuously draws expendedcoolant from the catheter at lower pressure and returns it at higherpressure to the coolant conditioner 40, thereby providing a closedcirculation loop through the catheter to meet the required ablation ormapping regimens. In the preferred embodiment, the conditioner providescoolant substantially at ambient temperature or colder, and thecontroller includes an electronically controlled pressure regulatorwhich sets the flow rate of the coolant injected into the catheter, thusregulating the cooling action of the catheter tip. Conditioned coolantis provided to the control section by the conditioner 40, which receivescoolant at lower pressure either from the return section 60 or from thesupply 30, compresses the coolant to a high pressure, liquefies thecoolant, and brings it to approximately ambient temperature at itsoutlet line 42 a leading to the controller. As further shown in FIG. 2,the output from the compressor has a second branch 42 b in which excesscoolant is not further cooled, but is simply returned to the supply 30.

As noted above, conditioner section 40 in addition to the raising thepressure of the coolant supplied to the regulator for controlledinjection into the catheter, also conditions the temperature of the highpressure coolant. This is preferably done as shown in FIG. 2, by heatexchange between the inlet supply line 41 and the compressor outlet line42. As shown in the Figure, the compressor outlet line 42 is placed inheat exchange communication, for example via a condenser or heatexchanger 45 b, with the inlet line 41. In addition one output branch 42a of the outlet line 42 is placed in heat exchange communication, forexample via exchanger 45 a, with an upstream portion of the inlet line41. The compressor 43 operates to compress the coolant from a relativelylow pressure, preferably below several atmospheres, to a considerablyhigher pressure, e.g., 20 to 30 atmospheres as measured in its outletline 42. The material in line 42 is therefore heated by compression, andthe heat exchange with inlet line 41 serves to reduce the temperaturerise generated by compression. Furthermore, by providing only a portionof compressor output, namely the catheter-directed branch 42 a to theupstream, colder portion of the compressor inlet line 41, the catheterinjection supply of coolant is effectively brought to or near ambienttemperature or colder, while the downstream heat exchange effected inheat exchanger 45 b with the entire output of the compressor is cooledto a lesser extent, serving a more traditional function of liquefyingthe coolant output and enhancing the overall cooling capacity of thecompressed fluid. This ordered heat exchange arrangement providespreferentially greater cooling to the catheter-directed supply line,resulting in a stabilized catheter input over a broader range ofoperating cycles.

In FIG. 2 the high pressure return 42 b to the tank may be implementedwith a pressure regulator located in-line ahead of the tank inlet toassure that coolant is returned to the tank only when its use elsewherein the circulation loop is not required, and that the pressure in theline first builds up to a level higher than the current tank pressure.

Thus the system of the present invention provides a closed loop coolantcirculation system wherein coolant is conditioned for provision to theinlet of a control module which injects the coolant into a catheter, andthe coolant returns in a closed loop to provide a continuous circulationof fluid at ambient temperature or colder into the catheter.

FIG. 3 shows a prototype embodiment in greater detail, illustratingrepresentative valves and regulators for implementing a preferred closedloop coolant supply 200. The coolant supply, compressor, control andreturn portions of system 200 are numbered with numerals 230, 240, 250,and 260 corresponding to the related subassemblies 30, 40, 50 and 60 ofsystem 20. As shown in this embodiment, a refrigerant tank 231 equippedwith a magnetic sight glass 231 a to indicate fill level, suppliesrefrigerant through a needle valve 232 along line 233 to a downstreampressure regulator 235. The pressure regulator 235 converts the nominaltank pressure of several hundred pounds per square inch to a fixed levelof 14 psia to provide a constant supply pressure to the inlet line 241of the compressor. At this stage the refrigerant is boiling at atemperature of about −60° Fahrenheit. The vacuum recovery return line262 joins the refrigerant inlet 241 at this point.

The compressor inlet line 241 passes through heat exchanger 245 en routeto the compressor 243, and also passes through a condenser 244, so thelow pressure liquid in the inlet line 241 is heated by the hot vaporcoming out of the compressor, causing it to become a vapor. Thecompressor 243 takes the vapor and pressurizes it to about 400 psi. Thepressurized output passes along line 242 through dryers D and sightglass SG, after which the high pressure outlet line bifurcates into twobranches 242 b and 242 a. An upstream pressure regulator 246 in line 242b builds and maintains pressure in the high pressure output lineallowing the regulator to open and return excess refrigerant to the tank231 when the pressure reaches a preset level, of about 400 psi, which ishigher than the nominal tank pressure, e.g., 200 psi.

The second branch 242 a of the output line 242 passes through the heatexchanger 245 located in the upstream portion of the input line 241,where it is further cooled to provide a conditioned output to thecontroller 250, which as shown includes a motorized pressure regulator254. Pressure regulator 254 controls the flow rate of coolant providedalong line 251 to the inlet port of the catheter (illustratedschematically). By way of example, the pressure regulator 254 may becontrolled by a control microprocessor in the console to provide coolantat a pressure of 250 psi for a time interval of 2.5 minutes. Control isgenerally done by actuating the motor of regulator 254 to achieve adesired set point and leaving the regulator at that setting for theindicated time period. A zero to 500 psi pressure transducer 255 isplaced in line 251 to provide feedback signals for implementing thecontrol of the regulator 254, which may further employ feedback from thethermocouple in the catheter.

The foregoing values of pressure and duration are given by way ofexample only, and it will be understood that typical cooling regimensimplemented by the control console 120 (FIG. 1) may run from severalseconds to five minutes or more, and that the coolant pressures whichare varied to achieve a desired rate of heat transfer or effectivelesion depth may vary from the coolant pressure in the tank toapproximately the pressure of the compressor output line 242 a.Advantageously, the pressure in line 251 remains greater than thesaturation pressure of the refrigerant being used such that it does notstart to boil before it reaches the tip.

As further shown in FIG. 3, the return line 115 b from the catheterattaches to vacuum section 260, while a solenoid operated purge valve257 extends between the catheter inlet line 251 and the low pressurereturn line 262 from the vacuum scavenging system 260. It will beunderstood that purge valve 257 will typically be operated to bleed theinlet line when the catheter is first attached and the supply compressoror return pump, respectively, are operated.

The return line 115 b from the catheter passes via vacuum protectionsolenoid-operated valve 261 to a vacuum pump 265, which maintains avacuum in the range of 2 to 40 millibars in the return line, and whichincreases the pressure of the expended coolant vapor to approximately 15psi. At the outlet side of the vacuum pump a similar solenoid operatedprotection valve 261 a is provided together with a check ball, and anoil filter OF which prevents pump oil from contaminating the circulatingcoolant or depositing in the coolant valves, catheter passages or othercomponents. A filter, e.g., 0.5 μm, appears in the catheter inlet line251. The entire vacuum system may be isolated by the solenoid operatedprotection valves 261, 261 a, during start-up or during a sensedover-pressure or blood leakage condition, and a check valve 265 preventsany pressure build-up on the vacuum pressure side of the catheter in theevent of pump or compressor failure, allowing coolant return directlyinto the return line 262 and compressor inlet 241. For this purpose, thecompressor output or various bypass or check valves 257, 264 are set apressure slightly higher than the output setting of the tank conditionerregulator 235, so that the coolant normally circulates into the catheterand through the vacuum system back into the compressor as a closed loop.

In the illustrated embodiment, a coolant refill port 275 is provided ata solenoid operated valve 277 in the compressor inlet line 241, allowinga refrigerant bottle attached at that point to employ the samecompressor 243 of the system to refill the supply tank 231. For thispurpose, a solenoid operated by-pass valve 237 is also supplied tobypass the upstream high pressure return regulator 246 between thecompressor output line 242 b and the tank, and speed up refill of thetank 231. Preferably, above the tank, a solenoid operated valve 238connects to a vent port to allow venting of any air which may haveaccumulated in the refrigerant tank due to leakage through the catheteror tubing. This vent is preferably controlled automatically by asuitable control program in the console 120. Venting may be implemented,for example, by providing a temperature sensor in the refrigerant tankand a pressure sensor at its top. Knowing the temperature of the liquidrefrigerant in the tank, the vent may be operated until the saturatedpressure is reached for the given refrigerant at the indicated tanktemperature. Such a venting step is to be performed each time theconsole is turned on. In addition to the foregoing elements, variouspressure indicators or temperature sensors may be situated along thedifferent lines to indicate operating parameters of the fluid therein.These are preferably sensors or indicators of the process control typewherein, rather than a dial display output, they provide an electricaloutput which connects to a microprocessor programmed to monitor thevarious conditions continuously to detect relevant safety, control ormaintenance conditions.

The invention being thus disclosed and described in illustrativeembodiments herein, variations and modifications as well as adaptationsof the invention to other systems will occur to those skilled in theart, and all such variations, modifications and adaptations areconsidered to lie within the scope of the invention as described hereinand defined in the claims appended hereto and equivalents thereof.

1. A cryoablation system comprising a cryotreatment catheter and acoolant console having: an inlet line; a reservoir of phase changecoolant; a supply line for supplying phase change coolant; a first meanscoupled to the supply line for providing the phase change coolant fromthe reservoir at elevated pressure along the inlet line to thecryotreatment catheter, wherein the first means conditions temperatureof the coolant at elevated pressure in the inlet line; a second meansfor recovering the phase change coolant from the cryotreatment catheterand raising its pressure; said first and second means, a portion of saidsupply line, and said cryotreatment catheter forming a supply loopexternal to the reservoir, the supply loop passing through thecryotreatment catheter, the first means being arranged in heat exchangecommunication with the supply line to condition the phase change coolantbefore it reaches the catheter along the inlet line so as to achieveeffective cooling regimens by controlling phase change coolant providedalong the inlet line while continuously recovering and recirculatingexpended coolant from the second means, a pressure regulator forcontrolling flow of coolant between the inlet line and the catheter, anda control microprocessor coupled to the pressure regulator andconfigured for setting the pressure regulator to effect a treatmentcycle.
 2. A coolant system for operation of a cryotreatment catheter totreat a patient, such system comprising: a reservoir of phase changefluid; a compressor for elevating the pressure of said fluid; at leastone heat exchanger for controlling the temperature and phase of theelevated pressure fluid; a microprocessor-controlled pressure regulatorfor setting a cryotreatment supply regimen of defined pressure andduration to supply an effective amount of conditioned fluid to a coolantport of the cryotreatment catheter, and a vacuum recovery assemblyconnectable to the cryotreatment catheter for continuously drawingexpended fluid from the catheter.